Electrical Power Systems Quality, Second Edition phần 9 potx

grounded Connected to earth or to some conducting body that serves in place Wiring and Grounding 439 *Reprinted from IEEE Standard 142-1991, IEEE Recommended Practice for Grounding of In

capability to electromechanical network relays In the past, these plemental relays had minimum time delays of 1 s or more since theirmission was to wait for the elevator to descend However, not all util-ities endorse this low-current, time-delay technique Some feel thatany time delay in opening the network protectors degrades the highservice quality that the network system is intended to provide.

sup-The load-generation control and DG tripping schemes mentionedabove are intended to ensure that the network protectors are neveropened by exported power As long as the schemes work properly, thenetwork protectors are never exposed to the out-of-phase voltage con-ditions that may exceed the switch capability However, because of thepotentially catastrophic consequences of causing a network protectorfailure, it is prudent to provide a backup An interlocking scheme thattrips the DG instantaneously when a certain number of network pro-tectors have opened ensures that the network protectors will not beexposed to out-of-phase voltages for more than a few cycles The deci-sion as to how many protectors must open before the DG is tripped(one, two, or all) is a tradeoff between security of the protectors andnuisance tripping of the DG Note that this scheme does not relieve the

DG installer from the responsibility of providing stuck-breaker backupprotection for the DG’s switching device

An even more secure approach to avoiding overstressing the networkprotectors is to replace existing protectors with new designs that are

capable of interrupting fault currents from sources with higher X/R

for higher currents

100 Current (% of transformer rating)

Figure 9.32 Adjustable reverse-power characteristic.

Distributed Generation and Power Quality

ratios and of withstanding out-of-phase voltages across the openswitch One major U.S manufacturer of network protector units hasrecently introduced such high-capacity protectors in 800- to 2250-A rat-ings and plans to introduce them in ratings up to 6000 A These pro-tectors are designed to be retrofitted in many existing types of networkunits.

A possible DG interconnection problem exists that would involve work protectors without a network bus interconnection If a DG isinterconnected on a feeder that also supplies a network unit, then if itsfeeder breaker is tripped and the DG is not rapidly isolated, it mayimpact one or more of the network units as if it were isolated on the net-work bus For this type of event to occur, the DG output does not have

net-to be matched net-to the feeder load For the excess generation case, it onlyhas to be momentarily greater than the load on the network bus Underthis condition the power continues to flow to the network bus from thefeeder with the interconnected DG, which keeps that protector closed.However, the excess power flows through the network back to the otherfeeders, resulting in the opening of the protectors connected to thosefeeders Once open, these protectors will be separating two indepen-dent systems For the case of less generation than load, the protectorconnecting to the feeder with the generation may trip Again, such acondition would have a protector separating two independent systems.Therefore, such DG applications should be avoided unless the DGbreaker is interlocked with the feeder breaker with a direct transfertrip scheme

9.7 Siting DG

The value of DG to the power delivery system is very much dependent

on time and location It must be available when needed and must bewhere it is needed This is an often neglected or misunderstood concept

in discussions about DG Many publications on DG assume that if 1

MW of DG is added to the system, 1 MW of additional load can beserved This is not always true

Utility distribution engineers generally feel more comfortable with

DG installed on facilities they maintain and control The obvious choicefor a location is a substation where there is sufficient space and com-munications to control centers This is an appropriate location if theneeds are capacity relief on the transmission system or the substationtransformer It is also adequate for basic power supply issues, and onewill find many peaking units in substations However, to provide sup-port for distribution feeders, the DG must be sited out on the feederaway from the substation Such generation will also relieve capacityconstraints on transmission and power supply In fact, it is more effec-

Distributed Generation and Power Quality 423

Distributed Generation and Power Quality

tive than the same amount of DG installed in the substation.Unfortunately, this generation is usually customer-owned and distrib-ution planners are reluctant to rely on it for capacity.

The application of DG to relieve feeder capacity constraints is trated in Fig 9.33 The feeder load has grown to where it exceeds alimit on the feeder This limit could be imposed by either current rat-ings on lines or switchgear It could also be imposed by bus voltage lim-its There is DG on the feeder at a location where it can actually relievethe constraint and is dispatched near the daily peak to help serve theload The straightforward message of the figure is that the load thatwould otherwise have to be curtailed can now be served Therefore, thereliability has been improved

illus-This application is becoming more common as a means to deferexpansion of the wire-based power delivery infrastructure The gener-ation might be leased for a peak load period However, it is more com-mon to offer capacity credits to customers located in appropriate areas

to use their backup generation for the benefit of the utility system Ifthere are no customers with DG in the area, utilities may lease space

to connect generation or, depending on regulatory rules, may providesome incentives for customers to add backup generation

There is by no means universal agreement that this is a permanentsolution to the reliability problem When utility planners are shownFig 9.33, most will concede the obvious, but not necessarily agree thatthis situation represents an improvement in reliability Three of thestronger arguments are

1 If the feeder goes out, only the customer with the DG sees animprovement in reliability There is no noticeable change in the ser-vice reliability indices

424 Chapter Nine

ON

Daily Load Profile

DG Sited to Provide Feeder Relief

Figure 9.33 DG sited to relieve feeder overload constraint.

Distributed Generation and Power Quality

2 Customer generation cannot be relied upon to start when needed.Thus, the reliability cannot be expected to improve.

3 Using customer-owned generation in this fashion masks the trueload growth Investment in wire facilities lags behind demand,increasing the risk that the distribution system will eventually not

be able to serve the load

It should also be noted that the capacity relief benefit is nullifiedwhen the distribution system is upgraded and no longer has a con-straint Thus, capacity credits offered for this application generallyhave a short term ranging from 6 months to 1 year

If one had to choose a location on the distribution feeder, whereshould the DG be located? The optimal DG siting problem is similar tothe optimal siting problem for shunt capacitor banks Many of the samealgorithms can be used with the chief difference being that the objectbeing added produces watts in addition to vars Some of the same rules

of thumb also apply For example, if the load is uniformly distributedalong the feeder, the optimal point for loss reduction and capacity relief

is approximately two-thirds of the way down the main feeder Whenthere are more generators to consider, the problem requires computerprograms for analysis

The utility does not generally have a choice in the location of connected DG The location is given for customer-owned generation,and the problem is to determine if the location has any capacity-relatedvalue to the power delivery system Optimal siting algorithms can beemployed to evaluate the relative value of alternative sites

feeder-One measure of the value of DG in a location is the additionalamount of load that can be served relative to the size of the DG.Transmission networks are very complex systems that are sometimesconstrained by one small area that affects a large geographical area Arelatively small amount of load reduction in the constrained areaallows several times that amount of load to be served by the system.This effect can also be seen on distribution feeders Because of thesimple, radial structure of most feeders, there is generally not a con-straint so severe that DG application will allow the serving of addi-tional load several times greater than the size of the generator.However, there can be a multiplying effect as illustrated in Fig 9.34.This example assumes that the constraint is on the feeder ratherthan on the substation If 1 MW of generation were placed in the sub-station, no additional load could be served on the feeder because nofeeder relief has been achieved However, if there is a good site on thefeeder, the total feeder load often can grow by as much as 1.4 MW This

is a typical maximum value for this measure of DG benefit on radialdistribution feeders

Distributed Generation and Power Quality 425

Distributed Generation and Power Quality

Another application that is becoming common is the use of DG tocover contingencies Traditionally, utilities have built sufficient wire-based delivery capacity to serve the peak load assuming one major fail-ure (the so-called N-1 contingency design criterion) At the distributionfeeder level, this involves adding sufficient ties to other feeders so thatthe load can be conveniently switched to an alternate feeder when afailure occurs There must also be sufficient substation capacity toserve the normal load and the additional load expected to be switchedover during a failure This results in substantial overcapacity when thesystem is in its normal state with no failures.

One potentially good economic application of DG is to provide port for feeders when it is necessary to switch them to an alternatesource while repairs are made Figure 9.35 depicts the use of DGlocated on the feeder for this purpose This will be substantially lesscostly than building a new feeder or upgrading a substation to coverthis contingency

sup-The DG in this case is located near the tie-point between two feeders

It is not necessarily used for feeder support during normal conditionsalthough there would often be some benefits to be gained by operatingthe DG at peak load When a failure occurs on either side of the tie, theopen tie switch is closed to pick up load from the opposite side The DG

is dispatched on and connected to help support the backup feeder.Locating the DG in this manner gives the utility additional flexibil-ity and more reconfiguration options Currently, the most common DGtechnology used for this application is currently diesel gensets Thegensets may be mounted on portable trailers and leased only for thepeak load season when a particular contingency leaves the system vul-nerable One or more units may be interconnected through a pad-

mounted transformer and may also employ a recloser with a DG tection relay This makes a compact and safe interconnection packageusing equipment familiar to utility personnel.

pro-9.8 Interconnection Standards

Standards for interconnection of DG to distribution systems are ined in this section Two examples illustrating the range of require-ments for interconnection protection are presented

exam-9.8.1 Industry standards efforts

There have been two main DG interconnection standards efforts in theUnited States IEEE Standard 929-20005 was developed to addressrequirements for inverters used in photovoltaic systems interconnectedwith utility systems The standard has been generally applied to alltechnologies requiring an inverter interface One of the main issuesthis standard addresses is the anti-islanding scheme The basic idea is

to introduce a destabilizing signal into the switching control so that itwill quickly drift in frequency if allowed to run isolated while the con-trol thinks it is still interconnected Amid fears that vendors wouldindependently choose schemes that might cancel out each other, agree-ment was reached on a uniform direction to drive the frequency

Another, more contested effort has been the development of IEEEStandard P1547,10which has not been approved as of the time of thiswriting The intent is to develop a national standard that will apply tothe interconnection of all types of DG to both the radial and network dis-tribution systems Vendors, utilities, and end users have joined in thiseffort, which appears to be converging This draft standard addressesmany of the issues described in this chapter, and the approach takenhere is largely consistent with the contents of this document

9.8.2 Interconnection requirements

The basic requirements for interconnecting DG to the utility tion system are listed here

distribu-Distributed Generation and Power Quality 427

Figure 9.35 DG sited near the tie-point between two feeders to help support

contingencies.

Distributed Generation and Power Quality

Voltage regulation. DG shall not attempt to regulate voltage whileinterconnected unless special agreement is reached with the utility Aspointed out previously, this generally means that the DG will operate

at a constant power factor or constant reactive power output acceptable

to the operation of the system Inverters in utility-interactive modewould typically operate by producing a current in phase with the volt-age to achieve a particular power output level

Anti-islanding. DG shall have relaying that is capable of detectingwhen it is operating as an island and disconnect from the power sys-tem Inverters should be compliant with IEEE Standard 929-2000 suchthat they would naturally drift in frequency when isolated from theutility source Relaying to detect resonant conditions that might occurshould be applied in susceptible DG applications

Fault detection. DG shall have relaying capable of detecting faults onthe utility system and disconnecting after a time delay of typically 0.16

to 2.0 s, depending on the amount of deviation from normal DG shoulddisconnect sufficiently early in the first reclose interval to allow tem-porary faults to clear (The utility may have to extend the first recloseinterval to ensure that this can be accomplished.) However, to preventnuisance tripping of the DG, the tripping should not be too fast The0.16-s (10 cycles at 60 Hz) delay is to allow time for faults on the trans-mission system or adjacent feeder to clear before tripping the DG need-lessly

Settings proposed for voltage and frequency relays for this tion are given in Table 9.1.10The cutoff voltages are nominal guidelinesand may have to be modified for some applications A common adjust-ment is to decrease the voltage trip levels to avoid nuisance tripping forfaults on parallel feeders For example, faults on parallel feeders willsometimes give voltages less than 50 percent, requiring the setting on

applica-428 Chapter Nine

TABLE 9.1 Typical Voltage and

Frequency Relay Settings for DG

Interconnection for a 60-Hz System

Condition Clearing time, s

the 10-cycle trip to be reduced to perhaps 40 percent The frequencytrip settings may be adjusted according to local standards Some utili-ties may want larger DG to remain connected to a much lower fre-quency (e.g., 57 Hz) to help with system stability issues following loss

of a major generating plant or a tie-line

Direct transfer trip (optional). For applications where it is difficult todetect islands and utility-side faults, or where it is not possible to coor-dinate with utility fault-clearing devices, direct transfer trip should beapplied such that the DG interconnect breaker is tripped simultane-ously with the utility breaker Transfer trip is usually advisable when

DG is permitted to operate with automatic voltage control because thissituation is much more likely to support an inadvertent island.Transfer trip is relatively costly and is generally applied only on large

DG systems Two relaying schemes for meeting these requirements arepresented in Secs 9.8.3 and 9.8.4

9.8.3 A simple interconnection

The protection scheme shown in Fig 9.36 applies to small systems thatare not expected to be able to support islands by themselves There isnot universal agreement on what constitutes a “small” DG system.Some utilities draw the line at 30 kW, while others might restrict this

to less than 10 kW Some may allow this kind of interface protection for

Distributed Generation and Power Quality 429

SERVICE TRANSFORMER

sizes up to 100 kW, or more The two relaying functions shown areexpected to do most of the work even for large DG systems Large sys-tems have additional relaying to provide a greater margin of safety.Small DG systems would commonly be connected to the load bus atsecondary voltage levels There would not be a separate transformer,although there may be separate metering Overcurrent protection isprovided by molded case circuit breakers The main DG interface pro-tection functions are

1 Over/under (O/U) voltage (27/59 relay)

2 Over/under frequency (81 O/U relay)

These relays can be used to trip either the generator breaker or themain service breaker, depending on the desired mode of operation.Tripping only the generator leaves the load connected, and this is prob-ably the desired operation for most loads employing small cogeneration

or peaking generators However, the utility may require the mainbreaker to be tripped if the DG system is running when a disturbanceoccurs

The main service breaker would also be tripped if the DG system is

to be used for backup power so that the DG system can continue to ply the load off-line It should be noted that special controls (not shown

sup-in Fig 9.36) may be required for this transfer to occur seamlessly It isnot always easy to accomplish

The over/under voltage relay has the primary responsibility to detectutility-side disturbances There should be no frequency deviation untilthe utility fault interrupter opens If the fault is very close to the gen-erator interconnection point and the voltage sag is deep, the overcur-rent relaying may also see the fault This will depend on the capability

of the DG system to supply fault current The overcurrent breakers arenecessary for protecting the DG system in case of an internal fault.Once the distribution feeder is separated from the utility bulk powersystem, an island forms The voltage and frequency relays then work

in concert to detect the island One would normally expect the voltage

to collapse very quickly and be detected by the undervoltage relay Ifthis does not happen for some reason, the frequency should quicklydrift outside the narrow band expected while interconnected so that the

81 O/U relay would detect it

9.8.4 A complex interconnection

The second protection scheme described here represents the otherextreme from the simple scheme presented in Sec 9.8.3 Figure 9.37shows the key functions in an actual distribution-connected DG instal-lation that employs a primary-side recloser This is a relatively complex

430 Chapter Nine

Distributed Generation and Power Quality

interconnection protection scheme for a large synchronous generator.There are many other variant schemes that may also be applied, andthe reader is referred to vendors of DG packages whose literaturedescribes these in great detail.

A large DG installation on the distribution system would typicallycorrespond to generators in the 1- to 10-MW range Most generatorslarger than this will be interconnected at the transmission level andhave relaying similar to utility central station generation

Figure 9.37 shows the relays necessary for interface protection aswell as some of the relays necessary for generator protection Not all

Distributed Generation and Power Quality 431

Figure 9.37 Protection scheme for a large synchronous generator with

the functions that might be necessary for proper control of the tor, interlocking of breakers, etc., are shown This installation is com-prised of multiple generators connected identically.

genera-In this example, there is a primary-side utility breaker for whichutilities will typically use a common three-phase recloser This is a con-venient switchgear package for utilities to install and probably theleast costly as well The recloser comes with overcurrent relaying (notshown), and a separate DG relay package has been added that operatesoff a separate potential transformer This is the main breaker used toachieve or ensure separation of the generator(s) from the utility

The relaying elements in the system and their function are as lows

fol-Primary side

■ 27/59: standard under/over voltage relay. This serves as the mary means of fault and island detection This can be used to blockclosing of the breaker until there is voltage present on the utility sys-tem, or there may be a separate relay for that purpose

pri-■ 81 O/U: standard over/under frequency relay for islanding detection.

■ 47: negative-sequence voltage relay (optional). This is a backupmeans for detecting utility-side faults that can be more sensitive thanvoltage magnitudes in some cases Also, it helps prevent generatordamage due to unbalance, although there is another relay for thathere

■ 59I: instantaneous (peak) overvoltage. This is a supplementalislanding detection function This would be employed in cases whereferroresonance or other resonance phenomena are likely This wouldoccur when utility-side capacitors interact with the generator reac-tance Since such overvoltages can cause damage quickly, the timedelay is much shorter than for the other relays—but not so short that

it trips on utility capacitor-switching transients

■ 59N (or 59G): neutral or ground overvoltage. This relay is installed

in the corner of a broken delta connection on the potential former It is a supplemental fault and islanding detection relay func-tion that measures the zero-sequence voltage This would detectconditions in which the generator is islanded on an SLG fault It ismore necessary when the primary connection of the transformer isdelta or ungrounded-wye

trans-These relaying functions may be moved to the secondary side of the vice transformer if there is no high-side breaker The relays would thentrip the main breaker on the secondary side

ser-432 Chapter Nine

Distributed Generation and Power Quality

No reverse power (32) function is used at this interface because netexport is expected.

Generator side

■ 50/51: overcurrent relay. Responsible for tripping the main breakerfor faults within the generator system May also trip for faults on theutility system that the generator feeds Therefore, the time delaymust be coordinated with the other relays so that it does not tripinadvertently

■ 46 relay at transformer: negative-sequence current. Assists in thedetection of faults on the utility system, particularly open-phase con-ditions, and trips the main breaker (Generators have a separate 46relay.)

■ 25: synchronizing relay. Controls closing of the main breaker whenthe generators are being interconnected to the utility (This schemewould also require synchronous check relays on the individual gen-erators if they are to be interconnected separately.)

■ 40: loss of field relay.

■ 46: negative-sequence current. Protects the machine against sive unbalanced currents, which may result from an internal faultbut may also be due to unbalance on the utility system

exces-■ 50/51: overcurrent relays. Protects the generator against excessiveloads and faults on either side of the generator breaker

Readers might easily get the impression from the material in this ter that interconnecting a DG installation to the distribution system isfraught with Gordian knot–like entanglement power quality problems.However, few problems can be expected for most DG applications in thenear future while the total penetration is relatively low There is a sig-

chap-Distributed Generation and Power Quality 433

Distributed Generation and Power Quality

nificant amount of DG that can be accommodated without affecting theoperation of the distribution system, but there is a limit The grid is notinfinite in capacity.

As a general rule, problems begin to appear when the total nected DG capacity approaches 15 percent of the feeder capacity.11,12This might drop to as little as 5 percent of capacity on more rural feed-ers or be as high as 30 percent if the DG is clustered near the substa-tion Voltage regulation problems are often the first to appear, followed

intercon-by interference with the utility fault-clearing process, which includesconcerns for islanding

Changes can be made to accommodate nearly any amount of DG Asthe amount of DG increases, the simple, low-cost distribution systemdesign must be abandoned in favor of a more capable design It willalmost certainly be more costly, but engineers can make it work.Deciding who pays for it is another matter

In a future of massively distributed generation, as some see it, munications and control will be key Today, most of the control of dis-tribution systems is accomplished by local intelligence operatingautonomously Systems with high penetrations of DG would benefitgreatly from fast, interconnected communications networks This isone technology shift that must accompany the spread of DG if it is to besuccessful in contributing to reliable, high-quality electric power

com-9.10 References

1 H L Willis and W G Scott, Distributed Power Generation Planning and Evaluation,

Marcel Dekker, New York, 2000.

2 N Jenkins, R Allan, P Crossley, D Kirschen, G Strbac, Embedded Generation, The

Institute of Electrical Engineers, London, U.K., 2000.

3 W E Feero, W B Gish, “Overvoltages Caused by DSG Operation: Synchronous and

Induction Generators,” IEEE Transactions on Power Delivery, January 1986, pp.

258–264.

4 R C Dugan, D T Rizy, Harmonic Considerations for Electric Distribution Feeders,

ORNL/Sub/81-95011/4, Oak Ridge National Laboratory, U.S DOE, March 1988.

5 IEEE Standard 929-2000, Recommended Practice for Utility Interface of Photovoltaic Systems.

6 R C Dugan, T E McDermott, “Operating Conflicts for Distributed Generation on

Distribution Systems,” IEEE IAS 2001 Rural Electric Power Conference Record,

IEEE Catalog No 01CH37214, Little Rock, Ark., May 2001, Paper No 01-A3.

7 Electrical Distribution-System Protection, 3d ed., Cooper Power Systems, Franksville,

Wis., 1990.

8 R H Hopkinson, “Ferroresonance Overvoltage Control Based on TNA Tests of

Three-Phase Delta-Wye Transformer Banks,” IEEE Transactions on Power Apparatus and Systems, Vol 86, No 10, October 1967, pp 1258–1265.

9 D R Smith, S R Swanson, J D Borst, “Overvoltages with Remotely-Switched

Cable-Fed Grounded Wye-Wye Transformers,” IEEE Transactions on Power Apparatus and Systems, Vol PAS-94, No 5, September/October 1975, pp 1843–1853.

10 IEEE Standard P1547, Distributed Resources Interconnected with Electric Power Systems, Draft 8, P1547 Working Group of IEEE SCC 21, T Basso, Secretary.

434 Chapter Nine

Distributed Generation and Power Quality

11 Protection of Electric Distribution Systems with Dispersed Storage and Generation (DSG) Devices, Oak Ridge National Laboratory, Report ORNL/CON-123, September

1983.

12 R C Dugan, T E McDermott, D T Rizy, S Steffel, “Interconnecting Single-Phase

Backup Generation to the Utility Distribution System,” Transmission and Distribution Conference and Exposition, 2001 IEEE/PES, Vol 1, 2001, pp 486–491.

9.11 Bibliography

Dugan, R C., T E McDermott, G J Ball, “Distribution Planning for Distributed

Generation,” IEEE IAS Rural Electric Power Conference Record, IEEE Catalog No.

00CH37071, Louisville, Ky., May 7–9, 2000, pp C4-1–C4-7.

Engineering Handbook for Dispersed Energy Systems on Utility Distribution Systems,

EPRI Final Report, TR-105589, November 1995.

Integration of Distributed Resources in Electric Utility Systems: Current Interconnection Practice and Unified Approach, EPRI Final Report, TR-111489, November 1998.

“Interconnecting Distributed Generation to Utility Distribution Systems,” Short Course, The Department of Engineering Professional Development, University of Wisconsin— Madison, 2001.

Distributed Generation and Power Quality 435

Distributed Generation and Power Quality

Distributed Generation and Power Quality

Wiring and Grounding

Many power quality variations that occur within customer facilities arerelated to wiring and grounding problems It is commonly stated atpower quality conferences and in journals that 80 percent of all thepower quality problems reported by customers are related to wiringand grounding problems within a facility While this may be an exag-geration, many power quality problems are solved by simply tightening

a loose connection or replacing a corroded conductor Therefore, anevaluation of wiring and grounding practices is a necessary first stepwhen evaluating power quality problems in general

The National Electrical Code®(NEC®)* and other important standardsprovide the minimum standards for wiring and grounding It is oftennecessary to go beyond the requirements of these standards to achieve asystem that also minimizes the impact of power quality variations (har-monics, transients, noise) on connected equipment While the intent ofthis book is to concentrate on subjects that are more amenable to engi-neering analysis, the basic principles of wiring and grounding are pre-sented in this chapter to provide the reader with at least a fundamentalunderstanding of why things are done References are provided through-out the text for readers interested in further details

10.1 Resources

Selected definitions are presented here from the IEEE Dictionary (Standard 100), the IEEE Green Book (IEEE Standard 142), and the

NEC These are the fundamental resources on wiring and grounding.

The IEEE Green Book and the NEC provide extensive information on

proper grounding practices for safety considerations and proper systemoperation However, these documents do not address concerns forpower quality.

Power quality considerations associated with wiring and groundingpractices are covered in Federal Information Processing Standard

(FIPS) 94, Guideline on Electrical Power for ADP Installations (1983).

This is the original source of much of the information interpreted andsummarized here

The IEEE Emerald Book (ANSI/IEEE Standard 1100-1992, IEEE

Recommended Practice for Powering and Grounding Sensitive Electronic Equipment) updates the information presented in FIPS 94.

This is an excellent resource for wiring and grounding with respect topower quality issues and is highly recommended

Grounding guidelines to minimize noise in electronic circuits are also

covered in IEEE Standard 518, IEEE Guide for the Installation of

Electrical Equipment to Minimize Electrical Noise Inputs to Controllers from External Sources EPRI’s Wiring and Grounding for Power Quality (Publication CU.2026.3.90) provides an excellent summary of

typical wiring and grounding problems along with recommended tions Additional resources are provided in the Bibliography at the end

solu-of this chapter

10.2 Definitions

Some of the key definitions of wiring and grounding terms from thesedocuments are included here

IEEE Dictionary (Standard 100) definition*

grounding A conducting connection, whether intentional or accidental, bywhich an electric circuit or equipment is connected to the earth, or to someconducting body of relatively large extent that serves in place of the earth

It is used for establishing and maintaining the potential of the earth (or ofthe conducting body) or approximately that potential, on conductors con-nected to it; and for conducting ground current to and from the earth (or theconducting body)

IEEE Green Book (IEEE Standard 142) definitions*

ungrounded system A system, circuit, or apparatus without an intentionalconnection to ground, except through potential indicating or measuring devices

or other very high impedance devices

grounded system A system of conductors in which at least one conductor orpoint (usually the middle wire or neutral point of transformer or generatorwindings) is intentionally grounded, either solidly or through an impedance

grounded solidly Connected directly through an adequate ground connection

in which no impedance has been intentionally inserted

grounded effectively Grounded through a sufficiently low impedance such thatfor all system conditions the ratio of zero sequence reactance to positive sequencereactance (X0/X1) is positive and less than 3, and the ratio of zero sequence resis-tance to positive sequence reactance (R0/X1) is positive and less than 1

resistance grounded Grounded through impedance, the principal element ofwhich is resistance

inductance grounded Grounded through impedance, the principal element

of which is inductance

NEC definitions † Refer to Fig 10.1

grounding electrode The grounding electrode shall be as near as practicable

to and preferably in the same area as the grounding conductor connection tothe system The grounding electrode shall be: (1) the nearest available effec-tively grounded structural metal member of the structure; or (2) the nearestavailable effectively grounded metal water pipe; or (3) other electrodes (Section250-81 & 250-83) where electrodes specified in (1) and (2) are not available

grounded Connected to earth or to some conducting body that serves in place

Wiring and Grounding 439

*Reprinted from IEEE Standard 142-1991, IEEE Recommended Practice for Grounding of Industrial and Commerical Power Systems, copyright © 1991 by the

Institute of Electrical and Electronics Engineers, Inc The IEEE disclaims any bility or liability resulting from the placement and use in this publication Information is reprinted with the permission of the IEEE.

responsi-†Reprinted with permission from NFPA 70-1993, the National Electrical Code® , right © 1993, National Fire Protection Association, Quincy, Mass 02269 This reprinted material is not the complete and official position of the National Fire Protection Association on the referenced subject, which is represented only by the stan- dard in its entirety.

copy-Wiring and Grounding

grounding conductor, equipment The conductor used to connect the rent-carrying metal parts of equipment, raceways, and other enclosures to thesystem grounded conductor and/or the grounding electrode conductor at theservice equipment or at the source of a separately derived system.

noncur-grounding electrode conductor The conductor used to connect the ing electrode to the equipment grounding conductor and/or to the groundedconductor of the circuit at the service equipment or at the source of a separatelyderived system

ground-grounding electrode system Defined in NEC Section 250-81 as including: (a)

metal underground water pipe; (b) metal frame of the building; (c) encased electrode; and (d) ground ring When these elements are available,they are required to be bonded together to form the grounding electrode sys-tem Where a metal underground water pipe is the only grounding electrodeavailable, it must be supplemented by one of the grounding electrodes specified

feeder All circuit conductors between the service equipment of the source of

a separately derived system and the final branch circuit overcurrent device

GROUNDING-ELECTRODE

CONDUCTOR

NEC 250-26(b)

GROUNDING ELECTRODE NEC 250-26(c) EARTH OR SOME CONDUCTING MATERIAL

EQUIPMENT GROUNDING CONDUCTORS

G N

L1 LOAD

INSULATED NEUTRAL

METALLIC CONDUCTOR ENCLOSURE NEC 250-91(b)

SYSTEM OVERCURRENT PROTECTION SUPPLY

Figure 10.1 Terminology used in NEC definitions.

Wiring and Grounding

outlet A point on the wiring system at which current is taken to supply lization equipment.

uti-overcurrent Any current in excess of the rated current of equipment or the

capacity of a conductor It may result from overload, short circuit, or ground fault

panel board A single panel or group of panel units designed for assembly inthe form of a single panel; including buses, automatic overcurrent devices, andwith or without switches for the control of light, heat, or power circuits;designed to be placed in a cabinet or cutout box placed in or against a wall orpartition and accessible only from the front

separately derived systems A premises wiring system whose power isderived from a generator, a transformer, or converter windings and has nodirect electrical connection, including a solidly connected grounded circuit con-ductor, to supply conductors originating in another system

service equipment The necessary equipment, usually consisting of a circuitbreaker switch and fuses, and their accessories, located near the point ofentrance of supply conductors to a building or other structure, or an otherwisedefined area, and intended to constitute the main control and means of cutoff

of the supply

ufer ground A method of grounding or connection to the earth in which thereinforcing steel (rebar) of the building, especially at the ground floor, serves as

a grounding electrode

10.3 Reasons for Grounding

The most important reason for grounding is safety Two importantaspects to grounding requirements with respect to safety and one withrespect to power quality are

1 Personnel safety. Personnel safety is the primary reason that allequipment must have a safety equipment ground This is designed toprevent the possibility of high touch voltages when there is a fault in apiece of equipment (Fig 10.2) The touch voltage is the voltage betweenany two conducting surfaces that can be simultaneously touched by anindividual The earth may be one of these surfaces

There should be no “floating” panels or enclosures in the vicinity ofelectric circuits In the event of insulation failure or inadvertent appli-cation of moisture, any electric charge which appears on a panel, enclo-sure, or raceway must be drained to “ground” or to an object which isreliably grounded

2 Grounding to assure protective device operation. A ground faultreturn path to the point where the power source neutral conductor is

grounded is an essential safety feature The NEC and some local wiring

codes permit electrically continuous conduit and wiring device sures to serve as this ground return path Some codes require the con-

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duit to be supplemented with a bare or insulated conductor includedwith the other power conductors.

An insulation failure or other fault that allows a phase wire to makecontact with an enclosure will find a low-impedance path back to thepower source neutral The resulting overcurrent will cause the circuitbreaker or fuse to disconnect the faulted circuit promptly

NEC Article 250-51 states that an effective grounding path (the path

to ground from circuits, equipment, and conductor enclosures) shall

a Be permanent and continuous

b Have the capacity to conduct safely any fault current likely to beimposed on it

c Have sufficiently low impedance to limit the voltage to ground and

to facilitate the operation of the circuit protective devices in thecircuit

d Not have the earth as the sole equipment ground conductor

3 Noise control. Noise control includes transients from all sources.This is where grounding relates to power quality Grounding for safetyreasons defines the minimum requirements for a grounding system.Anything that is done to the grounding system to improve the noiseperformance must be done in addition to the minimum requirements

defined in the NEC and local codes.

The primary objective of grounding for noise control is to create anequipotential ground system Potential differences between differentground locations can stress insulation, create circulating ground cur-rents in low-voltage cables, and interfere with sensitive equipmentthat may be grounded in multiple locations

Ground voltage equalization of voltage differences between parts of

an automated data processing (ADP) grounding system is accomplished

in part when the equipment grounding conductors are connected to thegrounding point of a single power source However, if the equipmentgrounding conductors are long, it is difficult to achieve a constant poten-

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Line Neutral

Safety Ground

System Ground

Fault Load

Dangerous Touch Potential

Ungrounded Cabinet

Figure 10.2 High touch voltage created by improper grounding.

Wiring and Grounding

tial throughout the grounding system, particularly for high-frequencynoise Supplemental conductors, ground grids, low-inductance groundplates, etc., may be needed for improving the power quality These must

be used in addition to the equipment ground conductors, which arerequired for safety, and not as a replacement for them

10.4 Typical Wiring and Grounding

Problems

Sections 10.4.1 to 10.4.7 describe some typical power quality problemsthat are due to inadequacies in the wiring and grounding of electricalsystems It is useful to be aware of these typical problems when per-forming site surveys because many of the problems can be detectedthrough simple observations Other problems require measurements ofvoltages, currents, or impedances in the circuits

10.4.1 Problems with conductors and

connectors

One of the first things to be done during a site survey is to inspect theservice entrance, main panel, and major subpanels for problems withconductors or connections A bad connection (faulty, loose, or resistive)will result in heating, possible arcing, and burning of insulation Table10.1 summarizes some of the wiring problems that can be uncoveredduring a site survey

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TABLE 10.1 Problems with Conductors and Connectors

Problem observed Possible cause

Burnt smell at the panel, Faulted conductor,

junction box, or load bad connection, arcing, or

Panel or junction box Faulty circuit breaker

is warm to the touch or bad connection

Buzzing (corona effect) Arcing

Scorched insulation Overloaded wiring, faulted

conductor, or bad connection

No voltage at load Tripped breaker, bad connection,

equipment or faulted conductor

Intermittent voltage at Bad connection or arcing

10.4.2 Missing safety ground

If the safety ground is missing, a fault in the equipment from the phaseconductor to the enclosure results in line potential on the exposed sur-faces of the equipment No breakers will trip, and a hazardous situa-tion results (see Fig 10.2)

neu-concern) This is a direct violation of the NEC.

10.4.4 Ungrounded equipment

Isolated grounds are sometimes used due to the perceived notion ofobtaining a “clean” ground The proper procedure for using an isolatedground must be followed (see Sec 10.5.5) Procedures that involve hav-ing an illegal insulating bushing in the power source conduit andreplacing the prescribed equipment grounding conductor with one to

an “isolated dedicated computer ground” are dangerous, violate code,and are unlikely to solve noise problems

10.4.5 Additional ground rods

Ground rods should be part of a facility grounding system and nected where all the building grounding electrodes (building steel,metal water pipe, etc.) are bonded together Multiple ground rods can

con-be bused together at the service entrance to reduce the overall groundresistance Isolated grounds can be used for sensitive equipment, asdescribed previously However, these should not include isolatedground rods to establish a new ground reference for the equipment.One very important power quality problem with additional ground rods

is that they create additional paths for lightning stroke currents toflow With the ground rod at the service entrance, any lightning strokecurrent reaching the facility goes to ground at the service entrance andthe ground potential of the whole facility rises together With addi-tional ground rods, a portion of the lightning stroke current will flow onthe building wiring (green ground conductor and/or conduit) to reach

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the additional ground rods This creates a possible transient voltageproblem for equipment and a possible overload problem for the con-ductors.

10.4.6 Ground loops

Ground loops are one of the most important grounding problems inmany commercial and industrial environments that include data pro-cessing and communication equipment If two devices are grounded viadifferent paths and a communication cable between the devices pro-vides another ground connection between them, a ground loop results.Slightly different potentials in the two power system grounds can causecirculating currents in this ground loop if there is indeed a completepath Even if there is not a complete path, the insulation that is pre-venting current flow may flash over because the communication circuitinsulation levels are generally quite low

Likewise, very low magnitudes of circulating current can causeserious noise problems The best solution to this problem in manycases is to use optical couplers in the communication lines, therebyeliminating the ground loop and providing adequate insulation towithstand transient overvoltages When this is not practical, thegrounded conductors in the signal cable may have to be supplementedwith heavier conductors or better shielding Equipment on both ends

of the cable should be protected with arresters in addition to theimproved grounding because of the coupling that can still occur intosignal circuits

10.4.7 Insufficient neutral conductor

Switch-mode power supplies and fluorescent lighting with electronicballasts are widely used in commercial environments The high third-harmonic content present in these load currents can have a very impor-tant impact on the required neutral conductor rating for the supplycircuits

Third-harmonic currents in a balanced system appear in the sequence circuit This means that third-harmonic currents from threesingle-phase loads will add in the neutral, rather than cancel as is thecase for the 60-Hz current In typical commercial buildings with adiversity of switched-mode power supply loads, the neutral current istypically in the range 140 to 170 percent of the fundamental frequencyphase current magnitude

zero-The possible solutions to neutral conductor overloading include thefollowing:

■ Run a separate neutral conductor for each phase in a three-phase cuit that serves single-phase nonlinear loads

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■ When a shared neutral must be used in a three-phase circuit withsingle-phase nonlinear loads, the neutral conductor capacity should

be approximately double the phase conductor capacity

■ Delta-wye transformers (see Sec 10.5.6) designed for nonlinear loadscan be used to limit the penetration of high neutral currents Thesetransformers should be placed as close as possible to the nonlinearloads (e.g., in the computer room) The neutral conductors on the sec-ondary of each separately derived system must be rated based on theexpected neutral current magnitudes

■ Filters to control the third-harmonic current that can be placed atthe individual loads are becoming available These will be an alter-native in existing installations where changing the wiring may be anexpensive proposition

■ Zigzag transformers provide a low impedance for zero-sequence monic currents and, like filters, can be placed at various places alongthe three-phase circuit to shorten the path of third-harmonic currentsand better disperse them

har-10.5 Solutions to Wiring and Grounding

Problems

10.5.1 Proper grounding practices

Figure 10.3 illustrates the basic elements of a properly grounded trical system The important elements of the electrical system groundingare described in Secs 10.5.2 to 10.5.5

Building Grounding Electrode

Wiring and Grounding

10.5.2 Ground electrode (rod)

The ground rod provides the electrical connection from the power tem ground to earth The item of primary interest in evaluating theadequacy of the ground rod is the resistance of this connection Thereare three basic components of resistance in a ground rod:

sys-■ Electrode resistance. Resistance due to the physical connection ofthe grounding wire to the grounding rod

■ Rod-earth contact resistance. Resistance due to the interfacebetween the soil and the rod This resistance is inversely propor-tional to the surface area of the grounding rod (i.e., more area of con-tact means lower resistance)

■ Ground resistance. Resistance due to the resistivity of the soil inthe vicinity of the grounding rod The soil resistivity varies over awide range, depending on the soil type and moisture content

The resistance of the ground-rod connection is important because itinfluences transient voltage levels during switching events and light-ning transients High-magnitude currents during lightning strokesresult in a voltage across the resistance, raising the ground referencefor the entire facility The difference in voltage between the ground ref-erence and true earth ground will appear at grounded equipmentwithin the facility, and this can result in dangerous touch potentials

10.5.3 Service entrance connections

The primary components of a properly grounded system are found atthe service entrance The neutral point of the supply power system isconnected to the grounded conductor (neutral wire) at this point This

is also the one location in the system (except in the case of a separatelyderived system) where the grounded conductor is connected to theground conductor (green wire) via the bonding jumper The ground con-ductor is also connected to the building grounding electrode via thegrounding-electrode conductor at the service entrance For most effec-tive grounding, the grounding-electrode conductor should be exother-mically welded at both ends

The grounding-electrode conductor is sized based on guidelines in

the NEC (Section 250-94) NEC table 250-94 (reproduced in Table 10.2)

provides the basic guidelines

There are a number of options for the building grounding electrode

It is important that all of the different grounding electrodes used in abuilding are connected together at the service entrance The followingare permissible for use as grounding electrodes:

■ Underground water pipe (See NEC table 250-94 for

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